Typical magnetic carrier for use in electrophotographic magnetic brush development along with toner is iron powder and ferrite particles having a resinous coating. The magnetic carrier is effective in triboelectrically charging the toner whereby the toner adheres to the carrier by an electrostatic force and then transferred to a photoconductor upon development.
Therefore, the magnetic carrier particles are required to have a sufficient triboelectric charge in a uniform manner to pick up the toner uniformly for subsequent deposition. The carrier particles should be efficient in carrying the toner in the developing unit and be free flowing powder.
Further, the carrier particles function as one electrode in the developing zone for producing a uniform electric field. They are thus required to have a desired resistance for a particular type of copying machine within the range of from 10.sup.5 to 10.sup.12 .OMEGA. by changing the composition of magnetic particles on which a resinous coating is applied or changing the composition of the resinous coating on magnetic particles. It is desired that the electric resistance of the carrier particles do not lower under humid conditions.
Another requirement imposed on the carrier is durability in that the carrier can maintain and perform its function consistently in the developing unit.
The prior art resinous coatings on magnetic particles are not satisfactory in all of these aspects.
The same assignee as the present invention proposed to use as a resinous coating composition an emulsion which is prepared by emulsion polymerizing a monoethylenic monomer in the presence of a polymerizable emulsifier (see Japanese Patent Application Kokai Nos. 270769/1986, 15561/1987 and 23054/1987). The carrier particles covered with such resinous coatings exhibit stable electric resistance and flow properties, have a controlled triboelectric charge, and are resistant against humidity.
However, these resin coated carrier particles still have several problems including a slow rise of triboelectric charging upon replenishment of toner, a change of electric charge with time, and the fusion of toner to carrier (toner spent). After repetitive copying operation over several ten thousand sheets, the image density will lower with time and toner scattering occur.